Actually, it isn't that bad of an idea. Aluminum foil is one of the only malleable alloys that are available to the public and won't kill you from toxicity. Aluminum is also a fabulous conductor and has some other neat properties.

Light is generally reflected from it, with some minor absorption for heat. Which makes it good for preserving food, AND cooking it. It can hold an electric charge like you wouldn't believe, and people use it all the time to make home-made resistors (even if its kinda old school).

It also is pretty resistant to natural solvents like water. Or fat. It is one of those eureka products that probably will be around forever. Like the wheel.

As far as hats go...it might actually work. Assuming you are trying to block some sort of electromagnetic radiation, you could get a benefit from wrapping foil around your head. It would also prevent you from spoiling. But the down side is...you'll heat up. Even just normal visible light will start heating up that foil until your head is a bacon ball.

Your choice though. I assure you, if an alien species has the technology to scan your brain or control your thoughts...tin foil wouldn't help. They would most likely have you breathe in nano-probes from ambient air. You wouldn't even know.

Well, having some knowledge of electronics and faraday cages, I have a major problem with the run-of-the-mill tinfoil hat. I've seen some studies done on them, and in some cases (depending on design and frequency) they can actually amplify the signals presented to them. I believe the major problem is that, unlike a true faraday cage, they are not grounded. I think to be useful, they would have to have a wire attached that hangs to the ground. This would also depend on the surface that wire was making contact with. The ground could also be run through the clothing and attached to a shoe with a metal plate on the bottom to complete the ground. Without a ground, the enclosure (hat) would be just as likely to bounce the signal around inside it as to short it out.

Well, having some knowledge of electronics and faraday cages, I have a major problem with the run-of-the-mill tinfoil hat. I've seen some studies done on them, and in some cases (depending on design and frequency) they can actually amplify the signals presented to them. I believe the major problem is that, unlike a true faraday cage, they are not grounded. I think to be useful, they would have to have a wire attached that hangs to the ground. This would also depend on the surface that wire was making contact with. The ground could also be run through the clothing and attached to a shoe with a metal plate on the bottom to complete the ground. Without a ground, the enclosure (hat) would be just as likely to bounce the signal around inside it as to short it out.